1. Field of the Invention
The present invention relates to image forming apparatuses by the electrophotographic technique, and mote particularly, to image forming apparatuses such as copying machines, printers, and facsimile machines.
2. Description of the Related Art
In a conventional image forming apparatus by the electrophotographic method, a fixing apparatus of a paired heat roller type with a heating roller and a pressure roller is normally employed. Also, a fixing apparatus with seamless belts as a heating member and a pressure member has been known.
In such a fixing apparatus that melts toner with heat generated by a heating member, heat is required for melting the toner. Accordingly, a large quantity of heat is necessary to increase the number of recording sheets on which fixing is performed in unit time (i.e., the productivity). As the basis weight of the recording material to be used is large, the quantity of heat required for fixing an image on the recording material increases. As a result, an even larger quantity of heat is required.
In a case where a full-color image is fixed as an output image, a large quantity of heat is required, as the amount of toner to be fixed is larger than the amount of toner required for mono-color printing, and the toner needs to be melted for a glossier image.
With an electrophotographic image forming apparatus, there is a demand for full-color image output with a productivity of 50 ppm or higher on heavy paper of a basis weight of 100 g/m2. In a case where an output is made on heavy paper with a high productivity, a large quantity of heat is necessary.
As the electric power that is used to meet the above demand increases, the quantity of heat accumulated on the heating roller increases. Especially, in a full-color fixing apparatus that needs to have an elastic layer covering the heating roller, the elastic layer serves as a heat insulating layer. Accordingly, the quantity of heat accumulated on the heating roller further increases, resulting in a high temperature of the core metal.
When a printing operation ends while the core metal of the heating roller is at a high temperature, the quantity of heat to be absorbed by the recording material suddenly loses a place to escape. As a result, the heat accumulated on the core metal increases the temperature of the entire roller, and an abnormal temperature rise is caused on the surface of the heating roller. With such an abnormal temperature rise, the following problems are observed.
That is, the first problem is that the toner to be originally fixed on the recording material is offset on the surface of the fixing roller, because the fixing operation is performed while the temperature of the fixing roller is very high. As a result, a defective image with a hot offset is caused, as the offset toner adheres to a recording material to be transferred to the fixing apparatus next.
So as to avoid the first problem, the printing operation may be prohibited and the apparatus may be put into a standby state until the temperature of the surface of the fixing roller drops to a predetermined temperature. However, because of the large capacity of heat, the waiting period might last for several minutes, which is undesirable in terms of usability. This is the second problem.
So as to eliminate the above problems, Japanese Unexamined Patent Publication No. 2004-102104 discloses a method by which the printing temperature control for the fixing apparatus is suspended before the fixing on the last page is completed, and the heating operation is then suspended or moved on to a rotation temperature control process. More specifically, in a case where the printer engine cannot recognize the number of pages formed by the image data, whether the operation should be performed is determined from the existence of data at the start of image formation at the first development station. If the number of pages in a job can be recognized, the timing of the operation is determined in advance based on the number of pages through reverse calculation started from the end of the fixing.
However, there are the following problems with the image forming apparatus of the above described prior art.
The technique disclosed in Japanese Unexamined Patent Publication No. 2004-102104 is effective for a fixing apparatus with a heating roller with a small quantity of heat and high thermal responsiveness. However, since a large-diameter heating roller is employed in a fixing apparatus that needs to exhibit a high productivity with heavy paper, the heat capacity is high and the thermal responsiveness is poor. As a result, a sufficient effect cannot be expected.
So as to secure fixability and glossiness on heavy paper in a belt-type fixing apparatus suitable for achieving a wide nip, the temperature of the pressure pad provided in the belt for preventing slippage of the belt is preferably maintained at a low temperature. Therefore, in a standby state in which printing is not performed, the pressure pad as well as the belt should preferably be separated from the heating roller.
However, after the belt is separated from the heating roller, the heat cannot escape from the heating roller to the belt. As a result, an abnormal temperature rise is caused in the heating roller for the above described reasons.